1. Field of the Invention
The present invention relates to a high strength screw such as a tapping screw, or a gib-head screw or bolt, which effects tightening and joining while forming a male thread in a opposing member in which a foundation hole has been formed.
2. Description of the Related Art
Screws such as tapping screws, and notably including gib-head screws and bolts, have hitherto been used as tightening and joining means in a variety of fields. These tapping screws are advantageous in that, by the simple forming of a foundation hole in an opposing member they create their own tapping as they are screwed, and so the advance work required to form a male thread in a opposing member, such as in the tightening and joining using normal machine screws (bolts and nuts), is saved, and the work can be markedly reduced.
Since a male thread must be formed in the opposing member in this way, the tapping screws must be sufficiently harder than the opposing member, and they must also possess the mechanical qualities required of a tightening and joining means (resistance to fatigue and resistance to impact and so on). For this reason, conventional tapping screws have normally been screws formed by the component rolling of a screw material comprising low carbon structure steel in which the surface thereof has been hardened by thermal processing. That is to say, the surface of the thread part of the tapping screw is a high carbon martensite that has a hardness equivalent to tool steel, and the core part is a low carbon martensite high in toughness. It should be noted that, since bolts and screws and the like are tension members, toughness is lost and strength is reduced in surface hardness processing, and it is commonly recognized that this is undesirable and that a thorough firing and bating (tempering) should be performed.
In addition, because conventional tapping screws form male threads in an opposing member, a large rotating force is required and so, although the fastening forces are sacrificed to a degree, the pitch of the thread part is enlarged to lessen the torque necessary for rotation. On the other hand. Although S Taito screws which have the same thread part pitch as a machine screw (xe2x80x9cS Taitoxe2x80x9d is a registered trade name of the Nitto Seiko K.K) are in existence, these are unsuitable for application involving tightening and joining in an opposing member because of their large tension. Generally, once conventional tapping screws have been removed from an opposing member in order for repairs or the like to be made they are unfit for refastening. That is to say, the screw thread of a tapping screw is subject to substantial damage as it is screwed in to create the tapping in a foundation hole of an opposing member, and the screw thread is also partly crushed by vibration and the like.
Because of the problems of material strength, surface hardening processing problems, and problems of workability such as this, there is no thought given to the re-tightening and joining of conventional tapping screws once they have been removed from tightening and joining with an opposing member, and consideration is given only to their workability at the time of initial tightening and joining, that is to say, to whether the male thread can be formed with little torque in the foundation hole. Up to this point, a description has been given of conventional examples and their problems taking tapping screws, which constitute a mode which has rigorous use, as an example, but the principal part of the above description holds true for general screws as well such as gib-head screws and bolts and the like.
It is an object of the present invention, in solving the problems in light of the above-described conditions, to provide a high strength screw in which a high strength steel material not used in the prior art is employed as the tapping screw or gib-head screw or bolt or the like, and moreover, which, by the administering of a carburization hardening process on the surface thereof, has high tensile stress and shear stress, and has high surface hardness, and which has, particularly in tapping screws, good male thread formability with respect to the foundation holes of the opposing member, and very good tightening and joining properties and durability. A further object of the present invention is to provide compatibility with a machine screw in such a way that, once the tapping screw has been removed from the opposing member, a machine screw can be screwed into the male thread formed by the tapping screw.
The present invention, in order to solve the above-described problems, is configured from a high strength screw formed from a screw material which comprises a high tensile steel, wherein a surface hardening process is performed on, at the least, the thread part, to form a surface hardened layer. Here, it is preferable that the abovementioned screw material be a type selected from carbon steel, aluminium killed steel, nickel-chromium-molybdenum steel, and chromium-molybdenum steel in which the carbon content is not less than 0.20%. In these cases, it is preferable that that the abovementioned surface hardening processing is continuous quenching gas carburization processing. Furthermore, it is preferable that the depth of said surface hardened layer is set within the range 0.05 to 0.3 mm by continuous quenching gas carburization processing.
In addition, the present invention is a high strength screw, wherein, when a nominal diameter of the screw is not more than 6 mm, said screw material contains carbon in an amount in the range 0.09 to 0.13%, manganese in an amount in the range 1.00 to 1.30%, aluminium in an amount in the range 0.02 to 0.05%, and the residual, apart from a plurality of metal elements contained in minute amounts, is iron, and the depth of said surface hardened layer is set within the range 0.05 to 0.3 mm by a quenching gas carburization process performed at a constant temperature; and when a nominal diameter of the screw is 8 to 12 mm, said screw material contains carbon in an amount in the range 0.15 to 0.18%, manganese in an amount in the range 1.00 to 1.50%, aluminium in an amount in the range 0.02 to 0.05%, and the residual, apart from a plurality of metal elements contained in minute amounts, is iron, and the depth of said surface hardened layer is set within the range 0.3 to 0.7 mm by a quenching gas carburization process performed at a constant temperature, and the surface hardness of the thread part is a Vickers hardness of 550 to 700 Hv, the hardness of the core part is a Vickers hardness of 200 to 320 Hv, and the tensile strength is 800 to 1200 N/mm2. Furthermore, the present invention is a high strength screw, wherein the structure of the depth is fine-divided and the high strength screw is formed which has both toughness as a tension member and a surface hardness of a degree that it will not be subjected to damage by the opposing member.
In addition, if the abovementioned screw is a tapping screw and the pitch of the abovementioned thread part is further set to a pitch which has compatibility with a machine screw, and furthermore, and it is preferable that the abovementioned tapping screw be an S Taito form (xe2x80x9cS Taitoxe2x80x9d is a registered trade name of the Nitto Seiko K.K) as once it is removed from, the opposing member after tightening and joining, re-tightening and joining can be effected by using the tapping hole formed initially, and a machine screw be used as a replacement and screwed in.
In addition, it is preferable that metal plating be performed on the surface of the abovementioned surface hardened layer to form a protective covering and to increase corrosion resistance.